A conventional power supplier of direct current such as that used in a portable personal computer is provided with a power diode and a transformer which are arranged in a plastic case. Usually, the heat generated by the diode is dissipated by using a heat sink attached on the diode, but an unfavorable dissipation efficiency is attained due to an insufficient area of cooling in the heat sink. Although a better dissipation efficiency may be achived by means of increasing the dimension of the heat sink, it in turn results in an increased dimension of the power supplier and the problem arises that the power supplier may not be carried handily.